Serially addressed sub-pupil screen for in situ electro-optical sensor wavefront measurement

ABSTRACT

A system and method for wavefront measurement of an EO sensor is performed in-situ using the sensor&#39;s EO detector in a manner that disambiguates the local wavefront measurements for different sub-pupils in time and maximizes the dynamic range for measuring the local wavefronts. A single sub-pupil sized optical beam is traced in a spatial pattern over the EO sensor&#39;s entrance pupil to serially illuminate a temporal sequence of sub-pupils to form a serially addressed sub-pupil screen. The EO detector and video card capture a video signal for one sub-pupil at a time as the optical beam traces the spatial pattern. The video signal is routed to a computer processor that generates a spatio-temporal mapping of the spatial positions of the sub-pupils in the sub-pupil screen to the temporal positions of frames in the video signal. The computer processor uses the mapping to process the video signal to compute a wavefront estimate spanning the entrance pupil.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the assembly and test of electro-optic (EO)sensors on a production line, and more particular to a test system, andmethod of wavefront measurement.

2. Description of the Related Art

Electro-optics (EO) sensors are configured to image scenes from a pointsource a long distance away, sufficiently far away that the receivedelectro-magnetic wavefront is typically considered to be planar. EOsensors are: typically used in SWIR, MWIR or LWIR bands but may alsooperate in the visible band. The EO sensor includes an opticaltelescope, transmissive or reflective, that is designed to focus anincident electro-magnetic wavefront to an image plane. In a typicalscene, the scene is composed of planar wavefronts from multiple pointsources. The structure of the scene is imprinted on the compositewavefront as a summation of planar wavefronts with different slopes. Thetelescope converts these slopes to spatial offsets in the image plane toform an image of the scene. An EO detector is mounted to the opticaltelescope at or near the image plane to convert the image of the sceneto an electronic representation. A video card reads out a temporalsequence of frames from the electronic representation to produce a videosignal at an output port. The video signal is provided to other systemssuch as a guidance module that process images of the scene.

Ideally the optical telescope converts the incident wavefront into aspherical wavefront that collapses onto the image plane of the opticalsystem. Given an ideal point source positioned on the optical axis ofthe telescope, any deviation from the perfect spherical wavefront (i.e.local slope differences of the wavefront) represents a wavefront errorthat distorts the image in some way and degrades system performance.Typical sources of error include surface shape defects in the opticaltelescope itself and mechanical stresses on the optical telescope frommounting the EO detector or other components. It is useful tocharacterize and understand these deviations in order to both qualify EOsensors during production and to mitigate the sources of error (e.g.improved alignment of telescope components, improved mounting of the EOdetector to the telescope and so forth). The wavefront measurement mayalso be used to directly compensate the errors via a deformable mirrorin some applications.

During production of an EO sensor or an assembly including an EO sensor,various tests and calibration procedures are performed at each stage ofassembly. Prior to any assembly, the bare optical telescope is tested tocompute an initial wavefront estimate. This test is typically performedwith an interferometer that superimposes a wavefront under test with areference wavefront. The difference between these wavefronts creates aninterference pattern with a series of fringes that can be mapped to thewavefront error.

The test may also be performed with a Shack-Hartman wavefront sensorthat illuminates the entire entrance pupil of the telescope with acollimated beam and uses another optic to image the wavefront onto alenslet array. The lenslet array spatially separates the wavefront intosub-pupils and focuses the sub-pupils simultaneously onto a detector.Each sub-pupil is focused onto a different sub-region on the detector,and the displacement of each sub-region image with respect to anexpected position from a desired wavefront can then be related to thelocal wavefront error. The extent of each sub-region defines the dynamicrange for measuring the local wavefront slope; the greater the spatialresolution the smaller the dynamic range. The different sub-regions ofthe detector are read out in parallel to provide the local wavefrontslope measurements across the entire wavefront simultaneously. Themeasurements are processed to compute a wavefront estimate. Theseestimation techniques are described by Harrison H. Barrett et al.“Maximum-likelihood methods in wavefront sensing: stochastic models andlikelihood functions” Vol. 24, No. 2/February 2007/J. Opt. Soc. Am. 1pp. 391-414. Shack-Hartman provides greater dynamic range for measuringlocal wavefront slopes (error) but less spatial resolution than theinterferometer.

The EO detector is then typically mounted onto the telescope near theimage plane to form the EO sensor. The EO sensor is subjected to avariety of tests and calibration procedures. If a test reveals a probleme.g. a focus test reveals that the EO sensor's modulation transferfunction (MTF) does not meet the specification, the unit is pulled offthe production line and retested using the interferometer orShack-Hartman wavefront sensor. In both cases, a collimated beam thatfills the entrance pupil is passed through the telescope and reflectedoff the EO detector back through the telescope to an external detector.In this double-pass configuration, alignment is critical, hencetime-consuming and expensive. Both the hardware and operation of theinterferometer and Shack-Hartman wavefront sensor are expensive. Bothrequire an external EO detector as part of the hardware package. Bothrequire an experienced engineer to perform the test.

SUMMARY OF THE INVENTION

The following is a summary of the invention in order to provide a basicunderstanding of some aspects of the invention. This summary is notintended to identify key or critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description and the defining claims that are presentedlater.

The present invention provides a system and method for wavefrontmeasurement of an EO sensor that is performed in-situ using the sensor'sEO detector in a manner that disambiguates the local wavefrontmeasurements for different sub-pupils in time and maximizes theachievable dynamic range of the local wavefront slope measurements.

In an embodiment, a single sub-pupil sized beam is traced over the EOsensor's entrance pupil to serially illuminate a temporal sequence ofsub-pupils in a spatial pattern to form a serially addressed “sub-pupilscreen”. The optical telescope focuses the single sub-pupil beam into asingle image at the image plane. An EO detector positioned at or nearthe image plane and video card capture a video signal of the image forone sub-pupil at a time as the beam traces the spatial pattern. Thevideo signal is routed to a computer processor that generates aspatio-temporal map that maps the spatial positions of the sub-pupils inthe sub-pupil screen to the temporal position frames in the videosignal. The computer processor uses the map to processes the videosignal and compute a wavefront estimate spanning the entrance pupil. Thewavefront estimate may be, computed using any number of estimationtechniques.

The single sub-pupil sized beam may be traced over the EO sensor'sentrance pupil in a variety of ways. In an embodiment, a collimatedbegin illuminates a pair of disks that are formed with complementaryhole patterns. Rotating the disks relative to each other causes only onesub-pupil sized hole in the disks to be aligned at a time to trace thesub-pupil sized collimated beam in a spatial pattern defined by the holepatterns. Peaks in the illumination of the EO detector provide a timingsignal for mapping positions of the sub-pupils in the sub-pupil screento a sub-sampled sequence of frames in the video signal. In anotherembodiment, sub-pupil sized collimated sources are positioned onconcentric circles of increasing radius with respect to an optical axisthrough the entrance pupil of the EO sensor. The EO sensor is rotatedabout the optical axis and the sources are activated one at a time insynch with the rotation of the sensor to trace the spatial pattern. Theprocessor estimates the sub-pupil position for each frame to mappositions of the sub-pupils in the sub-pupil screen to frames in thevideo signal. In another embodiment, a single sub-pixel collimated beammay be manually traced over the entrance pupil. In another embodiment, atwo-dimensional spatial light modulator may be used to selectively passa single sub-pupil of a pupil-sized collimated beam through themodulator to trace a single sub-pupil collimated beam over the entrancepupil. The sub-pupil beam need not be collimated, although for manystandard EO sensor's, it will be. All that is required is that thesub-pupil beam curvature is known and can be used to compare theresulting image with the current EO sensor in place to an image thatwould be formed with the desired optical system performance.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a test system using a serially addressedsub-pupil screen for in-situ EO sensor wavefront measurement;

FIG. 2 is a diagram of an embodiment of a serially addressed sub-pupilscreen

FIGS. 3 a and 3 b are diagrams of an image of a sub-pupil collimatedbeam in which mechanical stress on the telescope produces a shift in thecentroid of the image proportional to the local slope of the wavefront;

FIG. 4 is a diagram illustrating an embodiment of a spatio-temporal mapfrom the sub-pupils in the sub-pupil screen to the frames of the videosignal;

FIG. 5 is a diagram of an embodiment of a test system in which acomplementary pair of spinning disks trace a sub-pupil sized collimatedbeam to form the serially addressed sub-pupil screen;

FIGS. 6 a and 6 b are a drawing of an embodiment of a complementary pairof spinning disks and a time sequence illustrating the tracing of thesub-pupil sized collimated beam;

FIG. 7 is a diagram illustrating an embodiment of a spatio-temporal mapfrom the sub-pupils in the sub-pupil screen to the frames of the videosignal for the complementary pair of spinning disks;

FIG. 8 is a diagram of an embodiment of a test system in which aspinning EO sensor is synchronized to a line of sub-pixel sizedcollimated beams to trace a sub-pupil sized collimated beam to form theserially addressed sub-pupil screen;

FIGS. 9 a and 9 b are a drawing of an embodiment of the spinning EOsensor and collimated sources and a time sequence illustrating thetracing of the sub-pupil sized collimated beam; and

FIG. 10 is a diagram illustrating an embodiment of a spatio-temporal mapfrom the sub-pupils in the sub-pupil screen to the frames, of the videosignal for rotating EO sensor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a system and method for wavefrontmeasurement of an EO sensor that is performed in-situ using the sensor'sown EO detector. The described approach reduces the time and expenseassociated with the wavefront measurement by allowing the EO sensor tobe tested on the production line. The approach improves the quality ofthe wavefront estimate by disambiguating each of the sub-pupilmeasurements and by providing the maximum dynamic range for each localslope measurement for a given EO detector. This effectively decouplesthe sub-pupil size, hence spatial resolution of the wavefrontmeasurement from the dynamic range for which the wavefront can bemeasured without ambiguity.

Referring now to FIGS. 1, 2, 3 a-3 b and 4, an embodiment of a testsystem 10 provides for in-situ wavefront measurement of an electro-optic(EO) sensor 12 during test and calibration of the EO sensor in theproduction line. EO sensors are typically used in SWIR, MWIR or LWIRbands but may also operate in the visible band. The EO sensor 12includes an optical telescope 14, transmissive or reflective, single ormultiple element, that is designed to focus an electro-magneticwavefront to an image plane 16. In general, the image plane 16 is afunction of the focal length “f” of the telescope and the distance ofthe object to the front principal plane of the telescope. The objectdistance will determine the expected wavefront curvature at the entrancepupil of the system. In the specific and more typical case of an objectat an infinite (or very large) distance from the telescope, the objectis considered to be “collimated” and the image plane and focal plane ofthe system are coincident. Ideally, the optical telescope converts theincident wavefront into a spherical wavefront 18 that collapses onto theimage plane 16. In a typical scene, the scene is composed of planarwavefronts from multiple point sources at a very large distance from thetelescope. The structure of the scene is imprinted on the compositewavefront as a summation of planar wavefronts with different slopes. Thetelescope converts these different slopes to spatial offsets in theimage plane to form an image of the scene.

An EO detector 20 is mounted on the optical telescope by opto-mechanicalmounts 22 and positioned at or near the image plane 16 to convert theimage of the scene to an electronic representation. If the EO sensor isintended to image objects at a large distance in comparison to thetelescope focal length, the image plane and the focal plane arecoincident and thus the EO detector is placed not only at the imageplane, but also the focal plane of the telescope. The EO detector 20suitably comprises an x-y array of detector elements that are responsiveto illumination in the SWIR, MWIR, LWIR or visible bands. Each detectorelement measures and outputs the total illumination on that element fora specified sampling or integration period. A video card 24 reads out atemporal sequence of frames 26 from the electronic representation toproduce a video signal 28 at an output port 30. During ordinaryoperation, an internal cable 32 routes the video signal 28 from outputport 30 to a processing module 32 such as a guidance module.

The aperture of an optical system such as optical telescope 14 is thephysical object (e.g. the primary reflector of the telescope) thatlimits the bundle of light that propagates through the system from anobject on the optical axis (typically the centerline of the opticalsystem, assuming a rotationally symmetric optical system). The entrancepupil is the image of the aperture stop in object space (to the left ofthe optical telescope 14) and the exit pupil is the image of the stop inimage space (to the right of the optical telescope 14). A sub-pupil is aregion of the pupil smaller than the entire entrance pupil.

To perform the wavefront measurement, an optical source 38 traces asingle sub-pupil sized beam of light 40 in a spatial pattern 46 over theEO sensor's, entrance pupil 42 to serially illuminate a temporalsequence of sub-pupils 44 forming a serially addressed “sub-pupilscreen” 48. Typically, the sub-pupil screen exists in collimated spaceand the screen can be positioned anywhere between the collimated sourceand the entrance pupil 42. The optical source 38 can emulate any mannerof wavefront slope desired, but in most cases it is desired to emulate aplanar wavefront with zero slope produced by a single point source. Inthis case optical source 38 is considered to be collimated. The size ofthe sub-pupil should be at least ten times the wavelength to avoiddiffractive effects and no greater than one-quarter the size of theentrance pupil to provide adequate spatial resolution. More typicallyfor an IR sensor, the sub-pupil will be between ⅛ and 1/16 the size ofthe entrance pupil to adequately resolve the wavefront measurement.

The optical telescope 14 focuses the single sub-pupil beam 40 into asingle image 50 (e.g. a spot) at the image plane 16. The beam 40illuminates a sub-pupil region of the entrance pupil and is mapped tothe exit pupil of the system, modified in size by the ratio of the exitpupil and entrance pupil sizes. The sub-pupil of light is then directedfrom the exit pupil in a direction orthogonal to the wavefront, formingthe image 50 of the point source at the image plane of the EO sensor.Any optical errors (e.g. induced by mechanical stress) in theilluminated sub-pupil region of the entrance pupil will produce a shiftof the image 50 in the image plane. If the wavefront error can not beaccurately estimated by a simple slope change across the sub-pupil, theshape of the image 50 may change as well.

EO detector 20 converts image 50 to an electronic representation.Assuming no errors, each image 50 will be nominally located in thecenter of EO detector 20. The errors associated with each sub-pupil willproduce a different shift e.g. direction or magnitude. Because the sizeof the sub-pupil is smaller than the entrance pupil, the F-number of thesub-pupil is greater than that of the entrance pupil. A larger F-numbermeans that the central lobe of the diffraction pattern (i.e. image 50)is at least a few pixels wide (e.g. 3 or more) in each direction.Multiple pixels are preferred to get an accurate estimate of theposition of the central lobe.

Video card 24 captures the video signal 28 of the image 50 for onesub-pupil at a time as the beam 40 traces the spatial pattern and routesthe video signal to a computer processor. In an embodiment, the videosignal is routed from output port 30 via an external cable 52 to anexternal computer 54, where the signal is stored in memory and processedby the computer's processor. In another embodiment, the video signal isrouted from output port 30 via internal cable 32 to processing module 34that executes software to compute the wavefront estimate.

Because the EO detector 20 is positioned at or near the image plane, allof the sub-pupils in the sub-pupil screen are nominally focused to thecenter of the EO detector. Any shift of image 50 is due solely to errorsin the illuminated sub-pupil region of the entrance pupil. Consequently,the frames of the video signal must be mapped to the sub-pupil positionsin the sub-pupil screen to compute the wavefront estimate.

As shown in FIG. 4, the computer processor generates a spatio-temporalmapping 56 that maps the spatial positions of the sub-pupils 44 in thesub-pupil screen 48 to the temporal positions of frames 26 in the videosignal 28. Knowing the initial position of the sub-pupil collimatedbeam, the spatial pattern to be traced and the rate of tracing thepattern, the computer processor can map each sub-pupil 44 to aparticular frame 26 (or frames) in the video signal 28. Depending on theoptical source used to trace the sub-pupil beam, each sub-pupil 44 maybe captured in one and only one frame or a sequence of multiple frames.Some frames may not capture a sub-pupil, or may capture a partialsub-pupil that may be discarded.

Once the frames of the video signal have been mapped to the individualsub-pupils in the sub-pupil screen and stored in memory, the computerprocessor processes the data to compute a wavefront estimate 58 spanningthe entrance pupil. Once the mapping is complete, the processing of thedata to compute the wavefront estimate is or can be the same as thebackend processing for the Shack-Hartmann wavefront sensor.

The computation of wavefront estimates can follow any number of pathssuggested by estimation theory. The general case is to solve an inverseproblem to fit a parameterized description of the wavefront to themeasured sub-pupil data. A standard method for solving an inverseproblem is to create a model of the system under test (typicallyreferred to as a “forward” model), including the parameterizeddescription of the wavefront. The parameters are then varied until thedata in the model represents the collected data within the tolerancerequired for the estimate. There are many such algorithms that willarrive at a suitable answer, one of which is the Maximum LikelihoodEstimation Method. The MLE method is preferred by many because it isknown to be efficient if an efficient estimator exists (i.e. there is nobias in the estimate and the variance of the estimate has reached theCramer-Rao lower bound).

To improve the efficiency of computation, the computation of thewavefront estimate can be broken up into pieces; measuring localwavefront slopes for sub-pupils and integrating them to provide thewavefront estimate across the entrance pupil. The local wavefront slopesmay be obtained from the relation between the centroid of the imagedsub-pupil beam and the slope of the wavefront. The key is to make thesub-pupil small enough that the characteristics of the wavefront in thatregion can be estimated via a constant slope. If the wavefront ischanging slope rapidly the centroid measurement is no longer necessarilydirectly related to the characteristics of the wavefront over thesub-pupil. The centroid of each imaged sub-pupil beam is then comparedto where it is expected to be if the desired wavefront is incident. Ifthe sub-pupil beam is collimated, the difference in x and y from thedesired wavefront is recorded and related to the slope of the wavefrontfor it's sub-aperture via a simple optics relationship(x,y)=f(x,y)*tan(θ_(x),θ_(y)), where f is the focal length of the optic(in general could be different in x and y), x,y is the centroid andθ_(x), θ_(y) are the local slopes. The slopes of the wavefront at eachsub-pupil are integrated to form an estimate of the wavefront across thepupil.

If desired, an additional step of relating the integrated wavefront to aparameterized version of the wavefront (e.g. a polynomialrepresentation) might be performed at this point. The polynomials oftenchosen for convenience in optics are referred to as the Zernikepolynomials. In order to perform this properly it is important to assumethat none of the energy from an image formed at one sub-pupil crosses aboundary into an image for another sub-pupil. If this occurs, there willbe errors in the wavefront estimation.

Given that the goal of the data collection and analysis is to determinethe wavefront error across the entire pupil based on a sample of dataacross that pupil, this problem falls into the generic area ofmathematics, called estimation theory or inverse problems. In thatlight, one can see that separating the sub-pupil wavefront measurementsin time eliminates the opportunity for cross-contamination of thesesub-pupil measurements. Cross-contamination occurs in the Shack-Hartmannconfiguration as the local wavefront error increases. As this occurs,the individual sub-pupil images begin to merge together, and ultimatelya dynamic range limit on the wavefront measurement is reached. Inmathematical terms, this means that the Cramer-Rao lower bound on thevariance of the estimate is significantly increased when the sub-pupilmeasurements are ambiguous. By disambiguating the measurements in time,the lower bound on the variance of the estimate is reduced, leading to alarge increase in dynamic range.

With reference again to FIGS. 3 a and 3 b, these figures illustratecertain aspects of the described approach. First, the EO sensor's own EOdetector 20 is used to perform the test rather than an additionalexternal detector. This reduces the overall cost of the test systemsubstantially, particularly in the IR bands. Second, only one sub-pupilregion of the entrance pupil is illuminated at a given time. This hasthe desirable effect of disambiguating the measurements of the differentsub-pupils in time. The image from one sub-pupil does not interact withthe image from another sub-pupil. Third, the entire extent of the EOdetector 20 can be used to detect and measure the shift of image 50 foreach sub-pupil. This has the desirable effect of maximizing the dynamicrange of the measurement for each sub-pupil for a given EO detector.Consequently, the test system can measure larger errors and/or measurethe errors with greater resolution. Serially addressing the sub-pupilscreen effectively decouples spatial sampling resolution of thewavefront from the magnitude of the wavefront that can be measuredwithout ambiguity.

By comparison, the Shack-Hartman wavefront sensor cannot use thesensor's own EO detector because it requires a lenslet array at theplane of the entrance pupil. The only way to achieve this withoutaltering the system under test is to reimage the pupil with anadditional optic onto the Shack-Hartmann lenslet array that is coupledto an additional EO detector. If the errors are large enough, the imagesmay shift far enough to cause ambiguity between measurements foradjacent sub-regions. If energy from one sub-pupil crosses a boundaryinto a sub-region the detector assigned to a different sub-pupil therewill be errors in the wavefront estimation. The extent of eachsub-region defines the dynamic range for measuring the error in eachsub-pupil. Consequently the dynamic range is reduced by a factor equalto the number of sub-pupils in the pupil. The spatial resolution anddynamic range are directly coupled.

Referring now to FIGS. 5-7, an embodiment of a test system 100 uses acomplementary pair of spinning disks to trace a sub-pupil sized beamover the entrance pupil to serially illuminate sub-pupils. Test system100 includes a source 102 that projects a beam of light 104, a pair ofdisks 106 and 108 that have complementary hole patterns 110 and 112,respectively, typically positioned in collimated space, a rotation stage118 that rotates one of the disks about an axis 120 so that only onehole pair in the disks is aligned at a time to trace a single sub-pupilsized collimated beam 122 in a spatial pattern that spans the entrancepupil of an EO sensor 124 under test, and a computer processor 126 thatprocesses the EO sensor video signal to compute an estimate of thewavefront. Source 102 suitably includes a point source 128 that emits adiverging beam and an optic 130 that directs the light to form thedesired beam 104. EO sensor 124 includes an EO detector 132 that ismounted on an optical telescope 134 near the image plane and a videocard 136 coupled to the EO detector to generate the video signal.

In an embodiment of disks 106 and 108, hole pattern 110 includes aplurality of holes 140 of the sub-pupil size arranged in the spatialpattern to be traced and hole pattern 112 includes a plurality of holes142 of the sub-pupil size arranged in a complementary spatial patternwhere “complementary” means that rotation of the disks traces asub-pupil in a spatial pattern given by the hole pattern 110 on theother disk. As shown in FIG. 6 a, in an embodiment hole pattern 110comprises six arms 144 spaced around disk 106 at a first angle offsetfrom the optical axis 120 through the entrance pupil. Each arm 144includes multiple sub-pupil sized holes 140. Hole pattern 112 comprisesa single arm 146 at a second angle in opposition to the first angleoffset from the optical axis 120. Arm 146 includes multiple sub-pupilsized holes 142 equal in number to the number of holes in each arm 144.Note, this configuration is designed for a reflective optical telescopein which a center portion 148 of the entrance pupil is not illuminated,hence not characterized. As shown in FIG. 6 b, rotation of one of thedisks causes one hole pair 150 to be aligned at a time to trace a singlesub-pupil sized beam in a spatial pattern dictated by the hole pattern110 in disk 106.

As shown in FIG. 7, the spinning disks trace the sub-pupil beam to forma serially addressed sub-pupil screen 152 having six arms spaced aroundthe entrance pupil at an angle offset from the optical axis. Aspreviously described, knowing the initial positions of the two disks,the hole patterns of the two disks and the rate of rotation, thecomputer processor can map each sub-pupil 154 to a particular frame 156in the video signal 158.

The computer processor may improve the accuracy of this mapping byextracting a timing signal 160 from the video signal 158. As the disksrotate, the single hole pair varies between a state in which the holesare misaligned, perhaps completely, and a state in which the holes areperfect aligned. It follows that the amplitude of total illumination ofthe EO detector oscillates between a minimum amplitude when the holes inthe disks are misaligned and a peak amplitude when the holes in thedisks are aligned to form timing signal 160. The peak amplitudes 162 inthe timing signal 160 correspond to sub-pupils 154 in the sub-pupilscreen 152. Selection of the frames 156 corresponding to the peakamplitudes 162 creates a sub-sampled sequence 164 of frames. Forexample, every fourth frame (synchronized to the peak amplitudes) may besub-sampled from the complete video signal. The computer processor canthen accurately map the sub-pupils 154 to specific frames 156 in thesub-sampled sequence 164.

Referring now to FIGS. 8-10, an embodiment of a test system 200 in whicha spinning EO sensor is synchronized to a line of sub-pixel sizedoptical beams to trace a sub-pupil sized beam over the entrance pupil toserially illuminate sub-pupils. Test system 200 includes a line ofsub-pupil sources 202 positioned on concentric circles 204 of increasingradius with respect to an optical axis 206 through the entrance pupil ofthe EO sensor. Each said source suitably comprises a point source 208and an optic 210 and can be selectively turned on and off to produce asub-pupil sized beam of light 212. A rotation stage 214 rotates an EOsensor 216 (an EO detector 218 mounted on an optical telescope 220)about optical axis 206. The sources 208 are turned on one at a time insync with the rotation of EO sensor 216 to trace only one sub-pupilsized beam 212 in concentric circles 204 that span the entrance pupil toform a serially addressed sub-pupil screen 222 of sub-pupils 224.

EO sensor 216 includes a video card 226 that reads out a temporalsequence of frames 228 from the electronic representation produced bythe EO detector to produce a video signal 230. Knowing the positions ofthe sources, the activation sequence of the sources and rate of rotationof the EO sensor, a computer processor 232 estimates the sub-pupilposition for each frame to generate a spatio-temporal mapping of spatialpositions of the sub-pupils 224 in the sub-pupil screen 222 to temporalpositions of frames 228 in the video signal 230.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

We claim:
 1. A method of in-situ wavefront measurement of anelectro-optic (EO) sensor during test and calibration of the EO sensorin the production line, comprising: mounting the EO sensor in a testfixture, said EO sensor including an optical telescope having anentrance pupil for receiving an electro-magnetic wavefront and focusingthe wavefront to an image plane, an EO detector positioned near theimage plane to convert an image of a scene to an electronicrepresentation, a video card that reads out a temporal sequence offrames from the electronic representation to produce a video signal atan output port, and an opto-mechanical mount that mounts the EO detectorto the telescope; tracing a single sub-pupil sized optical beam in aspatial pattern over the entrance pupil to serially illuminate atemporal sequence of sub-pupils, said EO sensor's optical telescopefocusing the beam into an image on the EO detector, said EO detector andvideo card capturing a video signal of the image for one sub-pupil at atime as the beam traces the spatial pattern; routing the video signalfrom the output port to a computer processor; and with said computerprocessor computing a spatio-temporal mapping of the positions of thesub-pupils in the spatial pattern to the temporal positions of frames inthe video signal and using the mapping to compute a wavefront estimatespanning the entrance pupil.
 2. The method of claim 1, wherein producingthe video signal of the image of the optical beam for one sub-pupil at atime disambiguates local wavefront measurements for different sub-pupilsin time and maximizes the dynamic range for measuring the localwavefront by imaging the beam for one sub-pupil to the entire EOdetector.
 3. The method of claim 1, wherein the EO detector isresponsive to a band of electromagnetic energy in the SWIR, MWIR or LWIRbands.
 4. The method of claim 1, wherein the EO detector comprises anx-y array of detector elements, wherein a central lobe of the image ofthe sub-pupil optical beam spans at least three detector elements ineach of the x and y directions.
 5. The method of claim 4, wherein thesize of the sub-pupil sized optical beam is no greater than one-fourththe size of the entrance pupil.
 6. The method of claim 1, wherein thecomputer professor processes the frames to compute a centroid of theimage of the sub-pupil optical beam and from the centroid a local slopefor each sub-pupil and integrates the local slopes to compute thewavefront estimate.
 7. The method of claim 1, wherein the sub-pupiloptical beam is traced in the spatial pattern by, emitting a divergentbeam from a blackbody source; passing the divergent beam through acollimating telescope that produces a source collimated beam that spansthe entrance pupil; providing a first disk, said disk formed with aplurality of holes of the sub-pupil size arranged in the spatialpattern; providing a second disk, said disk formed with a plurality ofholes of the sub-pupil size arranged in a complementary spatial pattern;and rotating the second disk relative to the first disk so that only onehole in the second disk is aligned to only one hole in the first disk ata time and illuminated by the source collimated beam to trace the singlesub-pupil optical beam in the spatial pattern.
 8. The method of claim 7,further comprising: measuring an amplitude, of the illumination of theEO detector that oscillates between a minimum amplitude when the holesin the first and second disks are misaligned and a peak amplitude whenthe holes in the first and second disks are aligned to provide a timingsignal, said processor using the peak amplitudes of the timing signal tocreate a sub-sampled sequence of frames and mapping the positions of thesub-pupils in the spatial pattern to the sub-sampled sequence of frames.9. The method of claim 8, wherein the computer processor processes theframe of the video signal corresponding to the peak amplitude to computea centroid of the image of the collimated beam and a local slope foreach sub-pupil and integrates the local slopes to compute the wavefrontestimate.
 10. The method of claim 7, wherein the spatial patterncomprises multiple arms spaced around the first disk at a first angleoffset from an optical axis through the entrance pupil, each said armincluding multiple sub-pupil sized holes, and wherein said complementaryspatial pattern comprises a single arm at a second angle in oppositionto the first angle offset from the optical axis.
 11. The method of claim1, wherein the sub-pupil optical beam is traced in the spatial patternby, providing a line of sub-pupil collimated sources positioned onconcentric circles of increasing radius with respect to an optical axisthrough the entrance pupil of the EO sensor, each said collimated sourceconfigured to produce a sub-pupil sized collimated spot; rotating the EOsensor about the optical axis; and enabling the collimated sources oneat, a time in synch with the rotation of the EO sensor to trace only onesub-pupil sized collimated beam in concentric circles in the spatialpattern.
 12. The method of claim 11, wherein the computer processorestimates the sub-pupil position for each frame of the video signal togenerate the spatio-temporal mapping.
 13. The method of claim 12,wherein the computer processor processes each frame of the video signalto compute a centroid of the image of the collimated beam and a localslope for each sub-pupil and integrates the local slopes to produce thewavefront measurement.
 14. The method of claim 1, wherein the videosignal is routed to an external computer in which the computer processorresides.
 15. A method of in-situ wavefront measurement of anelectro-optic (EO) sensor during test and calibration of the EO sensorin the production line, comprising: mounting the EO sensor in a testfixture, said EO sensor including an optical telescope having anentrance pupil for receiving an electro-magnetic wavefront and focusingthe wavefront to an image plane, an EO detector comprising an x-y arrayof detector elements positioned near the image plane to convert an imageof a scene in an IR band to an electronic representation, a video cardthat reads out a temporal sequence of frames from the electronicrepresentation to produce a video signal at an output port, and anopto-mechanical mount that mounts the EO detector to the telescope;tracing a single sub-pupil sized optical beam no greater than one-fourththe size of the entrance pupil in a spatial pattern over the entrancepupil to serially illuminate a temporal sequence of sub-pupils to form asub-pupil screen that spans the entrance pupil, said EO sensor's opticaltelescope focusing the optical beam into an image on the EO detector,said image having a central lobe that spans at least three detectorelements in each of the x and y directions, said EO detector and videocard capturing a video signal of the image for one sub-pupil at a timeas the beam traces the spatial pattern; routing the video signal fromthe output port to a computer processor; and with said computerprocessor computing a spatio-temporal mapping of the spatial positionsof the sub-pupils in the sub-pupil screen to the temporal positions offrames in the video signal for computing a wavefront estimate.
 16. Atest system for in-situ wavefront measurement of an electro-optic (EO)sensor during test and calibration of the EO sensor in the productionline, comprising: a test fixture for mounting the EO sensor, said EOsensor including an optical telescope having an entrance pupil forreceiving an electro-magnetic wavefront and focusing the wavefront to animage plane, an EO detector positioned near the image plane to convertan image of a scene to an electronic representation, a video card thatreads out a temporal sequence of images from the electronicrepresentation to produce a video signal at an output port, and anopto-mechanical mount that mounts the EO detector to the telescope; anoptical source that traces a single sub-pupil sized optical beam in aspatial pattern over the entrance pupil to serially illuminate atemporal sequence of sub-pupils, said EO sensor's optical telescopefocusing the optical beam into an image on the EO detector, said EOdetector and video card capturing a video signal of the image for onesub-pupil at a time as the beam traces the spatial pattern; an externalcomputer having memory and a computer processor; and an external cableconnecting the EO sensor's video card output port to the externalcomputer to store the video signal in said memory, said computerprocessor generating a spatio-temporal mapping of the positions of thesub-pupils in the spatial pattern to the temporal positions of theframes in the video signal and processing the video signal to compute awavefront estimate spanning the entrance pupil, said serial illuminationdisambiguating local wavefront measurements of the wavefront in time andmaximizing the dynamic range for measuring the local wavefronts.
 17. Thetest system of claim 16, wherein the EO detector comprises an x-y arrayof detector elements, wherein the sub-pupil sized optical beam is nogreater than one-fourth the size of the entrance pupil so that a centrallobe of the image of the beam spans at least three detector elements ineach of the x and y directions.
 18. The test system of claim 16, whereinthe optical source comprises: a blackbody source that emits a divergentbeam; a collimating telescope that collimates the divergent beam toproduce a source collimated beam that spans the entrance pupil; a firstdisk formed with a plurality of holes of the sub-pupil size arranged inthe spatial pattern; a second disk formed with a plurality of holes ofthe sub-pupil size arranged in a complementary spatial pattern; and arotation stage that rotates the second disk relative to the first diskso that only one hole in the second disk is aligned to only one hole inthe first disk at a time and illuminated by the source collimated beamto trace the single sub-pupil sized optical beam in the spatial pattern.19. The test system 18, wherein said processor measures an amplitude ofillumination of the EO detector that oscillates between a minimumamplitude when the holes in the first and second disks are misalignedand a peak amplitude when the holes in the first and second disks arealigned to provide a timing signal, said processor using the peakamplitudes of the timing signal to create a sub-sampled sequence offrames and mapping the positions of the sub-pupils in the spatialpattern to the sub-sampled sequence of frames.
 20. The test system ofclaim 16, wherein the optical source comprises a line of sub-pupilcollimated sources positioned on concentric circles of increasing radiuswith respect to a longitudinal axis through the entrance pupil of the EOsensor, each said collimated source configured to produce a sub-pupilsized collimated spot, further comprising: a rotation stage for rotatingthe EO sensor about the longitudinal axis; and a controller forsynchronously enabling the collimated sources one at a time with therotation of the EO sensor to trace only one sub-pupil sized optical beamin concentric circles to form the spatial pattern.